Plumbing an ADU in San Jose: what to plan before you build

San Jose's ADU boom

San Jose issued just 10 ADU building permits in 2015. The city now reports 1,500 ADUs permitted since 2022 and more than 1,100 final building permits issued, so backyard units and garage conversions are now a common project rather than a rare one.

Every one of them needs plumbing: drains to the sewer, water from the main service and, often, gas. The cost of that plumbing depends heavily on decisions made early in the design.

Getting to the sewer

Most backyard ADUs connect to the main house's existing sewer lateral through a new wye and cleanout. That works when the lateral is in good condition and the ADU sits high enough for its drains to flow downhill to the connection.

Before tying in, have the existing lateral inspected with a camera. Connecting a new unit to a cracked or root-filled clay line only moves the problem. Where the slope does not work, a sewage ejector pump can lift the waste, at added cost and with ongoing maintenance.

Water for two households

Two kitchens and two or more bathrooms running at once can be more than an older water service was sized for. A plumber calculates the combined fixture demand and checks the meter and service line size, then decides whether a new branch is enough or the service needs upsizing.

The ADU should have its own shutoff, so work on one unit does not leave the other without water. San Jose's fire plan review may also ask for a water company letter showing hydrant flow, with 1,000 gallons per minute at 20 psi as the benchmark.

Gas or all-electric

PG&E now requires ADUs to have their own utility meter. If the ADU will use gas, the piping has to run to the new meter location and be sized for the unit's appliances. Many owners choose an all-electric ADU instead, with a heat pump water heater, which avoids gas piping altogether.

Either way, decide early, because it changes where the water heater goes, what the electrical panel needs and how much trenching is required.

Garage and junior ADU conversions

Garage conversions usually need the slab cut for new bathroom and kitchen drains, since garages rarely have drains in the right place. The shortest route to the existing sewer line keeps the work and the patching to a minimum.

Junior ADUs inside the existing house often tie into nearby drains and supply lines, which makes them simpler and faster to plumb than a detached unit.

Timing the plumbing with the build

Underground plumbing goes in and is inspected before the foundation or slab is poured. Rough-in inside the unit follows framing, and finish plumbing comes after drywall. The city's first round of ADU building review typically takes about 20 business days, so having the plumbing plan ready keeps the design moving.